Preheat oven to 350 degrees. Rinse green beans over cool water and set aside. In a large stock pot melt butter over medium high heat. Add garlic and onion and saute until translucent. Add flour and stir, creating a roux. Let the roux cook for 1 minute (stirring frequently) and pour in the cream. Whisk the cream thoroughly, mixing in the roux and reduce heat to medium low. Add the mushrooms and pulled pork and let cook for 15 minutes. Place green beans in a deep casserole dish and evenly pour over the cream mixture, coating the beans. Cover casserole with aluminum foil and bake for 20 minutes or until green beans are tender. While the casserole is baking line a cookie sheet evenly with frozen onion rings. Bake as directed. Top finished green bean casserole with crispy onion rings and season with salt and pepper to taste. Serve warm.